Mountains are the “water towers of the world.” In the Western U.S., 50 to 80 percent of the water supply originates in seasonal mountain snowpack. A growing challenge is maintaining aquatic ecosystem health while meeting human demand for water resources and ﬂood hazard protection. MSI studies the natural variability and human-caused change in water supply, water quality, and aquatic ecosystem health in the San Juan Mountains. MSI also supports snow science studies.